This refers to Cyan, Magenta, Yellow and BlacK (or Key), which are the subtractive primaries. They are pigments, which create a spectrum of colors in different combinations. Unlike monitors, printers use subtractive primaries to produce colors through subtractive mixing. The term “subtractive” is used because the primary colors are pure until you begin mixing them together, resulting in colors that are less pure versions of the primaries. You subtract values of C, M, Y and K to get white. You add values of R, G and B to get white.